The Solar System or solar system
comprises the Sun and
the retinue of
gravitationally bound to it: the eight
three currently identified
planets and their four known moons, and thousands of
small bodies. This last category includes
In broad terms, the charted regions of the Solar System consist of the Sun (astronomical
four rocky bodies close to it called the inner planets, an inner belt of
four giant outer planets and a second belt of small icy bodies known as
Kuiper belt. In order of their distances from the Sun, the planets are
Six of the eight planets are in turn orbited by
natural satellites (usually termed "moons" after Earth's
Moon) and every
planet past the
asteroid belt is encircled by
planetary rings of dust and other particles. The planets other than Earth
are named after gods and goddesses from
A whistle-stop tour of our solar system
From 1930 to 2006,
the largest known Kuiper
belt object, was considered the Solar System's ninth planet. However, in
International Astronomical Union (IAU) created an official
definition of the term "planet".
Under this definition, Pluto is reclassified as a
planet, and there are eight planets in the Solar System. In addition to
Pluto, the IAU currently recognizes two other
the largest object in the asteroid belt, and
Eris, which lies beyond the Kuiper belt in a region called the
Layout and structure
The principal component of the Solar System is the
G2 star that
contains 99.86% of the system's known
mass and dominates
Sun's two largest
orbiting bodies, account for more than 90% of the system's remaining mass.
(The currently hypothetical
would also hold a substantial percentage were its existence confirmed.)
Most objects in orbit around the Sun lie within the
shallow plane parallel to that of Earth's orbit. The planets are very close to
the ecliptic while
kuiper belt objects are usually at significantly greater angles to it.
All of the planets and most other objects also orbit with the Sun's rotation;
in a counter-clockwise direction as viewed from a point above the Sun's north
pole. (There are exceptions to this rule, a notable one being
There is a direct relationship between how far away a planet is from the Sun
and how quickly it orbits. Mercury, the closest to the Sun, travels the fastest,
while Neptune, being much farther from the Sun, travels more slowly. Objects
orbit in an
ellipse around the Sun, so an orbiting object's distance from the Sun varies
in the course of its year. Its closest approach to the Sun is known as its
while its farthest point from the Sun is called its
Although the orbits of the planets are nearly circular (with perihelions roughly
equal to their aphelions), many comets, asteroids and objects of the Kuiper belt
follow highly elliptical orbits with large differences between perihelion and
The paths of objects around the Sun travel according to a
law of planetary motion discovered by German astronomer
Johannes Kepler in the early 1600s. Under Kepler's laws, each planet orbits
along an ellipse with the Sun at one focus of the ellipse. However, Newton's
laws of motion dictate that just as the planets orbit around the Sun, so the Sun
is minutely affected by the gravity of the planets, and moves in a much, much
smaller eliptical trajectory about the focal point too. The primary focal point
in the Solar System is that between the Sun and Jupiter, since Jupiter is far
and away the largest of the planets. This point lies just outside the Sun itself
and is roughly equivalent to the Solar System's centre of mass, or barycentre.
Astronomers most often measure distances within the solar system in
astronomical units or AU. One AU is the approximate distance
between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi).
Pluto is roughly
39 AU from the Sun while
Jupiter lies at roughly 5.2 AU. One
the best known unit of interstellar distance, is roughly 63,240 AU.
Informally, the Solar System is sometimes divided into separate zones. The
first zone, known as the inner Solar System, includes the four
terrestrial planets and the main asteroid belt. The
outer Solar System is sometimes defined as "everything beyond the
asteroids". Alternatively, the term may be used to describe the region beyond
Neptune, with the four
considered a separate "middle zone".
One common misconception is that the orbits of the major objects within the
Solar System (planets, Pluto and asteroids) are equidistant. To cope with the
vast distances involved, many representations of the Solar System simplify these
orbits by showing them the same distance apart. However, in reality, with a few
exceptions, the Solar System is arranged so that the farther a planet or belt is
from the Sun, the larger the distance between it and the previous orbit. For
Venus is approximately 0.33 AU farther out than
Jupiter is 1.9 AU from the farthest extent of the
asteroid belt and Neptune's orbit is roughly 20 AU farther out than that of
Uranus. Attempts have been made to determine a correlation between these
but to date there is no accepted theory that explains the orbital distances.
Planets, dwarf planets, and small solar system bodies
In a decision passed by the
International Astronomical Union General Assembly on
24 August 2006, the objects
in the Solar System other than the Sun and
natural satellites were divided into three separate groups: planets,
dwarf planets and small solar system bodies.
Under this classification, a
any body in orbit around the Sun that a) has enough mass to form itself into a
spherical shape and b) has cleared its immediate neighborhood of all smaller
objects. Eight objects in the Solar System currently meet this definition; they
are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Dwarf planet was a second and new classification. The key difference
between planets and dwarf planets is that while both are required to orbit the
Sun and be of large enough mass that their own gravity pulls them into a nearly
round shape, dwarf planets are not required to
clear their neighborhood of other celestial bodies. Three objects in the
solar system are currently included in this category; they are
considered a planet), the asteroid
scattered disc object
The IAU will begin evaluating other known objects to see if they fit within the
definition of dwarf planets. The most likely candidates are some of the larger
asteroids and several Trans-Neptunian Objects such as
The remainder of the objects in the Solar System were classified as
small solar system bodies (SSSBs). As the IAU noted in its
The current hypothesis of Solar System formation is the
nebular hypothesis, first proposed in 1755 by
Immanuel Kant and independently formulated by
The nebular theory holds that 4.6 billion years ago (a date determined via
radiometric dating of
the Solar System formed from the gravitational collapse of a gaseous cloud. This
initial cloud was likely several light-years across and probably played host to
the births of several stars.
Although the process was initially viewed as relatively tranquil, recent studies
of ancient meteorites reveal traces of elements only formed in the hearts of
very large exploding stars, indicating that the environment in which the Sun
formed was within range of a number of nearby supernovae. The shock wave from
these supernovae may have triggered the formation of the Sun by creating regions
of overdensity in the surrounding nebula, causing gravitational forces to
overcome their internal gas pressure, and thus, them in turn to collapse.
One of these regions of collapsing gas (known as the
nebula) would form what became the Sun. This region had a diameter of
between 7000 and 20,000 AU
and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).
As the nebula collapsed, conservation of
angular momentum meant that it spun faster. As the material within the
nebula condensed, the atoms within it began to collide with increasing
frequency, causing them to release energy as heat. The centre, where most of the
mass collected, became increasingly hotter than the surrounding disc.
As the competing forces associated with gravity, gas pressure, magnetic fields,
and rotation acted on it, the contracting nebula began to flatten into a
protoplanetary disk with a diameter of roughly 200 AU
and a hot, dense
at the center.
stars, young, pre-fusing solar mass stars believed to be similar to the Sun
at this point in its evolution, show that they are often accompanied by discs of
These discs extend to several hundred AU and are rather cool, reaching only a
thousand kelvins at their hottest.
Eventually, the temperature and pressure at the core of the Sun became so
great that the hydrogen began to fuse, creating an internal source of energy
which countered the force of gravitational contraction. At this point the Sun
became a fully fledged star.
From the remaining cloud of gas and dust, the various planets formed. The
currently accepted method by which the planets formed is known as
accretion, in which the planets began as dust grains in orbit around the
central protostar, which initially formed by direct contact into clumps between
one and ten kilometres in diameter, which in turn collided to form larger bodies
of roughly 5 km in size gradually increasing by further collisions by roughly 15
cm per year over the course of the next few million years.
The inner solar system was too warm for volatile molecules like water and
methane to condense, and so the planetesimals which formed there were relatively
small (comprising only 0.6% the mass of the disc)
and composed largely of compounds with high melting points, such as
and metals. These
rocky bodies eventually became the
terrestrial planets. Farther out, the gravitational effects of
it impossible for the protoplanetary objects present to come together, leaving
Farther out still, beyond the
frost line, where more volatile icy compounds could remain solid,
able to gather more material than the terrestrial planets, as those compounds
were more common. They became the
captured much less material and are known as ice giants because their cores are
believed to be made mostly of ices (hydrogen compounds).
After 100 million years, the pressure and density of hydrogen in the centre
of the collapsing nebula became great enough for the
thermonuclear fusion, which increased until
hydrostatic equilibrium was achieved.
The young Sun's
below) then cleared away all the gas and dust in the
protoplanetary disk, blowing it into interstellar space, thus ending the
growth of the planets. T-Tauri stars have far stronger
winds than more stable, older stars.
The Sun is
the Solar System's parent star, and far and away its chief component. Its large
mass gives it an interior
enough to sustain
nuclear fusion, releasing enormous amounts of
energy, most of
radiated into space in the form of
electromagnetic radiation including visible
light. It is
classed as a moderately large
dwarf; however, this name is misleading, as on the scale of stars in our
galaxy, the Sun is rather large and bright. Stars are classified based on their
position on the
Hertzsprung-Russell diagram, a graph which plots the brightness of stars
against their surface temperatures. Generally speaking, the hotter a star is,
the brighter it is. Stars which follow this pattern are said to be on the
sequence, and the Sun lies right in the middle of it. This has led many
astronomy textbooks to label the Sun as "average;" however, stars brighter and
hotter than it are rare, whereas stars dimmer and cooler than it are common. The
vast majority of stars are dim
though they are under-represented in star catalogues as we can observe only
those few that are very near the Sun in space.
The Sun's position on the
sequence means, according to current theories of stellar evolution, that it
is in the "prime of life" for a star, in that it has not yet exhausted its store
of hydrogen for
nuclear fusion, and been forced, as older
must, to fuse more inefficient elements such as
carbon. The Sun
is growing increasingly bright as it ages. Early in its history, it was roughly
75 percent as bright as it is today.
Calculations of the ratios of hydrogen and helium within the Sun suggest it is
roughly halfway through its life cycle, and will eventually begin moving off the
main sequence, becoming larger and brighter but also cooler and redder, until,
about five billion years from now, it too will become a red giant.
The Sun is a
population I star, meaning that it is fairly new in galactic terms, having
been born in the later stages of the universe's evolution. As such, it contains
more elements heavier than hydrogen and helium ("metals" in astronomical
parlance) than older
population II stars such as those found in
Since elements heavier than hydrogen and helium were formed in the cores of
ancient and exploding stars, the first generation of stars had to die before the
universe could be enriched with them. For this reason, the very oldest stars
contain very few metals, while stars born later have more. This high
metallicity is thought to have been crucial in the Sun's developing a
planetary system, because planets form from accretion of metals.
The Sun radiates a continuous stream of charged particles, a
plasma known as
ejecting it outwards at speeds greater than 2 million kilometres per hour,
creating a very tenuous atmosphere (the
heliosphere), that permeates the solar system for at least 100 AU. This
environment is known as the
interplanetary medium. The influence of the Sun's rotating magnetic field on
the interplanetary medium creates the largest structure in the solar system, the
heliospheric current sheet.
magnetic field protects its atmosphere from interacting with the solar wind.
However, Venus and Mars do not have magnetic fields, and the solar wind causes
their atmospheres to gradually bleed away into space.
The interplanetary medium is home to at least two disclike regions of
dust. The first, which lies in the inner solar system, is known as the
zodiacal dust cloud and is responsible for the phenomenon of
zodiacal light. It was likely formed by collisions within the asteroid belt
brought on by interactions with the planets.
The second, which extends from about 10 AU to about 40 AU, was probably created
by similar collisions within the Kuiper belt.
The four inner or
terrestrial planets are characterised by their dense,
rocky composition, few or no moons, and lack of ring systems. They are
composed largely of minerals with high melting points, such as the
which form their solid
crusts and semi-liquid
mantles, and metals such as
cores. Three of the four inner planets (Venus, Earth and Mars) have
substantial atmospheres; all have
impact craters and possess tectonic surface features such as
volcanoes. The term inner planet should not be confused with
inferior planet, which designates those planets which are closer to the
Sun than the Earth is (i.e. Mercury and Venus).
The four inner planets are:
Mercury (0.4 AU), the closest planet to the Sun, is also the smallest of
the planets, at only 0.055 Earth masses. Mercury is very different from the
other terrestrial planets; it has no
natural satellite, and its only known geological features besides
impact craters are "wrinkle ridges" probably produced by a period of
contraction early in its history.
Its almost negligible atmosphere consists of atoms blasted off its surface by
the solar wind.
Its relatively large iron core and thin mantle have not yet been adequately
explained. Hypotheses include that its outer layers were stripped off by a giant
impact, and that it was prevented from fully accreting by the young Sun's
(0.7 AU) is of comparable mass to the
Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate
mantle around an iron core, as well as a substantial
atmosphere and evidence of internal geological activity, such as
However, it is much drier than Earth and its atmosphere is 90 times as dense.
Venus has no natural satellite. It is the hottest planet, with surface
temperatures over 400 °C, most likely due to the amount of
greenhouse gases in the atmosphere.
Although no definitive evidence of current geological activity has yet been
detected on Venus, its substantial atmosphere and lack of a magnetic field to
protect it from depletion by the solar wind suggest that it must be regularly
replenished by volcanic eruptions.
The largest and densest of the inner planets,
Earth (1 AU)
is also the only one to demonstrate unequivocal evidence of current geological
activity. Earth is the only planet known to have
life. Its liquid
hydrosphere, unique among the terrestrial planets, is probably the reason
Earth is also the only planet where
plate tectonics has been observed, because water acts as a lubricant for
Its atmosphere is radically different from the other terrestrial planets, having
been altered by the presence of life to contain 21 percent free oxygen.
It has one satellite, the
Moon; the only
large satellite of a terrestrial planet in the Solar System.
(1.5 AU), at only 0.107 Earth masses, is smaller than Earth and Venus. It
possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with
vast volcanoes and rift valleys such as
Valles Marineris, shows that it was once geologically active and recent
suggests this may have been true until very recently. Mars possesses two tiny
Phobos) thought to be captured
small solar system bodies that are composed in significant part of rocky and
metallic non-volatile minerals.
asteroid belt occupies the orbit between Mars and Jupiter, between 2.3
and 3.3 AU from the Sun. It is thought to be the remnants from the Solar
System's formation that failed to coalesce because of the gravitational
interference of Jupiter. Asteroids range in size from hundreds of kilometers to
as small as dust. All asteroids save the largest,
small solar system bodies; however, a number of other asteroids, such as
could potentially be reclassed as
planets if it can be conclusively shown that they have achieved
hydrostatic equilibrium. The asteroid belt contains tens of thousands - and
potentially millions - of objects over one kilometre in diameter.
However, despite their large numbers, the total mass of the main belt is
unlikely to be more than a thousandth of that of the
In contrast to its various depictions in
science fiction, the main belt is very sparsely populated; spacecraft
routinely pass through without incident. Asteroids with diameters between 10 and
10-4 m are called
Ceres (2.77 AU) is the largest astronomical body in the asteroid belt
and the only known
planet in this region. It has a diameter of slightly under 1000 km, large
enough for its own gravity to pull it into a spherical shape. Ceres was
considered a planet when it was discovered in the nineteenth century, but was
reclassified as an asteroid in the 1850s as further observation revealed
It was again reclassified in 2006, and is now considered to be a dwarf planet.
Asteroids in the main belt are subdivided into
asteroid groups and
families based on their specific orbital characteristics.
Asteroid moons are asteroids that orbit larger asteroids. They are not as
clearly distinguished as planetary moons, sometimes being almost as large as
their partners. The asteroid belt also contains
which may have been the source of Earth's water.
Trojan asteroids are located in either of Jupiter's
or L5 points, (gravitationally stable regions leading and
trailing a planet in its orbit) though the term is also sometimes used for
asteroids in any other planetary Lagrange point as well.
asteroids are those Trojans whose orbits are in a 2:3 resonance with
Jupiter; that is, they go around the Sun three times for every two Jupiter
The inner solar system is also dusted with
rogue asteroids, many of which cross the orbits of the inner planets.
The four outer planets, or
Jovian planets) are so large they collectively make up 99 percent of the
mass known to orbit the Sun.
Saturn are true
giants, at 318 and 95 Earth masses, respectively, and composed largely of
hydrogen and helium.
both substantially smaller, being only 14 and 17 Earth masses, respectively.
Their atmospheres contain a smaller percentage of hydrogen and helium, and a
higher percentage of “ices”, such as
this reason some astronomers suggested that they belong in their own category,
“Uranian planets,” or “ice giants.”
All four of the gas giants exhibit orbital debris rings, although only the ring
system of Saturn is easily observable from Earth. The term outer planet
should not be confused with
superior planet, which designates those planets which lie outside
(thus consisting of the outer planets plus
(5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets
put together. Its composition of largely
and helium is
not very different from that of the Sun. Jupiter's strong internal heat creates
a number of semi-permanent features in its atmosphere, such as cloud bands and
Great Red Spot. The four largest of its 63
Galilean satellites) share elements in common with the terrestrial planets,
such as volcanism and internal heating.
Ganymede, the largest satellite in the Solar System, has a diameter larger than
(9.5 AU), famous for its extensive
ring system, has many qualities in common with Jupiter, including its
atmospheric composition, though it is far less massive, being only 95 Earth
masses. Two of its 56 moons,
Enceladus, show signs of geological activity, though they are largely
Titan, like Ganymede, is larger than
Mercury; it is also the only satellite in the solar system with a
(19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely
among the planets, it orbits the Sun on its side; its
lies at over ninety degrees to the
Its core is remarkably cold compared with the other gas giants, and radiates
very little heat into space.
Uranus has 27 satellites, the largest being
(30 AU), though slightly smaller than Uranus, is denser at 17 Earth masses, and
radiates more internal heat than Uranus, but not as much as Jupiter or Saturn.
Neptune has 13 moons. The largest,
Triton, is geologically active, with geysers of
and is the only large satellite to revolve around its host planet in a
motion. Neptune possesses a number of
small solar system bodies, usually only a few kilometres across, composed
largely of volatile ices and possessing highly eccentric orbits; generally
perihelion within the orbit of the inner planets and an
far beyond Pluto. When a comet approaches the Sun, its icy surface begins to
sublimate, or boil away, creating a
coma; a long tail of gas and dust which is often visible with the naked eye.
There are two basic types of comet: short-period comets, with orbits less
than 200 years, and long-period comets, with orbits lasting thousands of years.
Short-period comets, such as
Halley's Comet, are believed to originate in the
belt, while long period comets, such as
(pictured), are believed to originate in the
Some comets with
orbits may originate outside the solar system.
Old comets that have had most of their volatiles driven out by solar warming are
often categorized as asteroids.
The area beyond Neptune, often referred to as the
outer solar system or simply the "trans-Neptunian
region", is still largely unexplored.
This region's first formation is the
belt, a great ring of debris similar to the asteroid belt, but composed
mainly of ice and far greater in extent, extending between 30 and 50 AU from the
Sun. This region is thought to be the place of origin for short-period
comets, such as
Halley's comet. Though it is composed mainly of
small solar system bodies, many of the largest Kuiper belt objects, such as
2005 FY9 and
could soon be reclassified as dwarf planets. There are estimated to be over
Kuiper belt objects with a diameter greater than 50 km; however, the total
mass of the Kuiper belt is relatively low, perhaps only a tenth or even a
hundredth the mass of the Earth.
Many Kuiper belt objects have multiple satellites and most have orbits that take
them outside the plane of the ecliptic.
The Kuiper belt can be roughly divided into two regions: the "resonant" belt,
consisting of objects whose orbits are in some way linked to that of Neptune
(orbiting, for instance, twice for every three Neptune orbits, or once for every
two), which actually begins within the orbit of Neptune itself, and the
"classical" belt, consisting of objects that don't have any resonance with
Neptune, and which extends from roughly 39.4 AU to 47.7 AU.
Members of the classical Kuiper belt are classified as
after the first of their kind to be discovered,
Pluto and Charon
(39 AU average), is the largest known object in the
belt and was previously accepted as the smallest planet in the Solar System.
In 2006, it was reclassified as a
planet by the Astronomers Congress organized by the International
Astronomers Union (IAU).
Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane
and ranging from 29.7 AU from the Sun at
(within the orbit of Neptune) to 49.5 AU at
Prior to the
Charon was considered a moon of Pluto, but in light of the redefinition
it is unclear whether Charon will continue to be classified as a moon of Pluto
or as a dwarf planet itself. Charon does not exactly orbit Pluto in a
traditional sense; Charon is about one-tenth the mass of Pluto and the center of
gravity of the pair is not within Pluto. Both bodies orbit a
of gravity above the surface of Pluto (in empty space), making Pluto-Charon a
binary system. Two much smaller moons,
Hydra, orbit Pluto and Charon. Pluto lies in the resonant belt, having a 3:2
resonance with Neptune (ie, it orbits twice round the Sun for every three
Neptune orbits). Those Kuiper belt objects which share this orbit with Pluto are
Overlapping the Kuiper belt but extending much further outwards is the
scattered disc. Scattered disc objects are believed to have been
originally native to the Kuiper belt, but were ejected into erratic orbits in
the outer fringes by the gravitational influence of
Neptune's early outward migration. Most scattered disc objects have
perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun.
Their orbits are also highly inclined to the ecliptic plane, and are often
almost perpendicular to it. Some astronomers, such as Kuiper belt co-discoverer
Jewitt, consider the scattered disc to be merely another region of the
Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt
Eris (68 AU average) is the largest known
scattered disc object and was the cause of the most recent debate about
what constitutes a planet since it is at least 5% larger than Pluto with an
estimated diameter of 2400 km (1500 mi). It is now the largest of the known
It has one moon,
The object has many similarities with Pluto: its orbit is highly eccentric,
perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an
97.6 AU, and is steeply inclined to the ecliptic plane, at 44 degrees, more so
than any known object in the solar system except the newly-discovered object
(also known as "Buffy")
and is believed to consist largely of rock and ice.
Centaurs, which roughly extend from 9 to 30 AU, are icy comet-like
bodies that orbit in the region between Jupiter and Neptune. The largest known
10199 Chariklo, has a diameter of between 200 and 250 km.
The first centaur to be discovered,
Chiron, has been called a comet since it has been shown to develop a coma
just as comets do when they approach the sun.
Some astronomers class Centaurs as scattered Kuiper belt objects along with the
residents of the scattered disc, merely Kuiper belt objects scattered inward,
rather than outward.
The point at which the solar system ends and interstellar space begins is not
precisely defined, since its outer boundaries are delineated by two separate
solar wind and the
The solar wind extends to a point roughly 130 AU from the Sun, whereupon it
surrenders to the surrounding environment of the
interstellar medium. The Sun's gravity however, holds sway to almost halfway
to the next star system. The vast majority of the solar system therefore, is
completely unknown; however, recent observations of both the solar system and
other star systems have led to an increased understanding of what is or may be
lying at its outer edge.
The heliosphere expands outward in a great bubble to about 95 AU, or three
times the orbit of
Pluto. The edge of this bubble is known as the
termination shock; the point at which the solar wind collides with the
opposing winds of the
interstellar medium. Here the wind slows, condenses and becomes more
turbulent, forming a great oval structure known as the
heliosheath that looks and behaves very much like a comet's tail;
extending outward for a further 40 AU at its stellar-windward side, but tailing
many times that distance in the opposite direction. The outer boundary of the
is the point at which the solar wind finally terminates, and one enters the
environment of interstellar space.
Beyond the heliopause, at around 230 AU, lies the
a plasma "wake" left by the Sun as it travels through
Sedna is a large, reddish Pluto-like object with a gigantic, highly
elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at
aphelion and takes 12,050 years to complete.
Mike Brown, who discovered the object in 2003, asserts that it cannot be
part of the
scattered disc or the
Belt as it has too distant a
perihelion to have been affected by Neptune's migration. He and other
astronomers consider it to be the first in an entirely new population, one which
also may include the object
which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of
Sedna is very likely a dwarf planet, though its shape has yet to be determined
is a great mass of up to a trillion icy objects that is believed to be the
source for all long-period
comets and to
surround the solar system like a shell from 50,000 to 100,000 AU beyond the
Sun. It is believed
to be composed of comets which were ejected from the inward Solar System by
gravitational interactions with the outer planets. Because the Sun's
gravitational hold on them is so weak, Oort cloud objects move only very slowly,
though they can be perturbed by such rare events as collisions, or the
gravitational effects of a passing star or the
The solar system is located in the
barred spiral galaxy with a diameter estimated at about
100,000 light years
containing approximately 200 billion stars. Our Sun resides in one of the Milky
Way's outer spiral arms, known as the
or Local Spur.
While the orbital speed and radius of the galaxy are not accurately known,
estimates place the solar system at between 25,000 and 28,000 light years from
galactic center and its speed at about 220
kilometres per second, completing one revolution every
years. This revolution is known as the Solar System's
The solar system appears to have a very remarkable orbit. It is both
extremely close to being circular, and at nearly the exact distance at which the
orbital speed matches the speed of the compression waves that form the spiral
arms. Evidence suggests that the Solar System has remained between spiral arms
for most of the existence of life on Earth. The radiation from
in spiral arms could theoretically sterilize planetary surfaces, preventing the
formation of complex life, save perhaps in the deepest oceans. The solar system
also lies well outside the star-crowded environs of the galactic centre. The
opposing gravitational tugs from so many close stars within the galactic centre
would have prevented planets from forming.
The Solar apex, the direction of the Sun's path through interstellar space,
is near the constellation of Hercules in the direction of the current location
of the bright star Vega.
At the galactic location of the solar system, the
escape velocity with regard to the gravity of the Milky Way is at least 500
The immediate galactic neighborhood of the Solar System is known as the
Local Interstellar Cloud or Local Fluff; an area of dense cloud in an
otherwise sparse region known as the
Bubble, an hourglass-shaped cavity in the
interstellar medium roughly 300
light-years across. The bubble is suffused with high-temperature plasma that
suggests it is the product of several recent supernovae.
There are relatively few
stars within ten light years (95 trillion km) of the Sun. The closest is
the triple star system
Alpha Centauri, which is located roughly 4.4 light years away (the outlying
star of the triple, the
Proxima Centauri, is closer, at 4.22 light years). Alpha Centauri A and B
are a closely tied pair of Sun-like stars. The stars next closest to the Sun are
the red dwarfs
Barnard's Star (at 6 light years),
(7.8 light years) and
Lalande 21185 (8.3 light years). The largest star within ten light years is
dwarf star roughly twice the Sun's mass and orbited by a
It lies 8.6 light years away. The remaining systems within ten light years are
the binary red dwarf system
UV Ceti (8.7
light years) and the solitary red dwarf
(9.7 light years).
Our closest solitary sunlike star is
which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but
only 60 percent its luminosity.
For many years, the Solar System was the only known example of planets in
orbit around a star. The discovery in recent years of many
extrasolar planets has led to the term "solar system" being applied
generically to all the newly discovered systems. Technically, however, it should
strictly refer to Earth's system only, as the word "solar"
is derived from the Sun's
Sol. Other such
systems are usually referred to by the names of their parent star (for example
Alpha Centauri system" or "the
Recent studies of extrasolar systems neighboring Earth's have shown that our
system's configuration might not be common, as the vast majority so far
discovered have been found to be markedly different. For instance, many
extrasolar planetary systems contain a "hot
a planet of comparable size to Jupiter that nonetheless orbits very close to its
star, at, for instance, 0.05 AU. It has been hypothesised that while the giant
planets in these systems formed in the same place as the gas giants in Earth's
solar system did, some sort of migration took place which resulted in the giant
planet spiralling in towards the parent star. Any terrestrial planets which had
previously existed would presumably either be destroyed or ejected from the
system. On the other hand, the apparent prevalence of hot Jupiters could result
sampling error, as planets of similar size at greater distances from their
stars are more difficult to detect.
Discovery and exploration
For many thousands of years, people, with a few notable exceptions, did not
believe the Solar System existed. The Earth was believed not only to be
stationary at the centre of the
but to be categorically different from the divine or ethereal objects that moved
through the sky. While
Nicholas Copernicus and his predececessors, such as the Indian
and the Greek philosopher
Aristarchus of Samos, had speculated on a heliocentric reordering of the
cosmos, it was the conceptual advances of the 17th century, led by
Johannes Kepler, and
Newton, which led gradually to the acceptance of the idea not only that
Earth moved round the Sun, but that the planets were governed by the same
physical laws that governed the Earth, and therefore could be material worlds in
their own right, with such earthly phenomena as craters, weather, geology,
seasons and ice caps.
The first exploration of the solar system was conducted by telescope, when
astronomers first began to map those objects too faint to be seen with the naked
Galileo Galilei was the first to discover physical details about the
individual bodies of the Solar System. He discovered that the
Moon was cratered,
that the Sun was marked with sunspots, and that Jupiter had four satellites in
orbit around it.
Christiaan Huygens followed on from Galileo's discoveries by discovering
Titan and the shape of the
rings of Saturn.
Giovanni Domenico Cassini later discovered four more
moons of Saturn, the
Cassini division in Saturn's rings, and the
Great Red Spot of Jupiter.
Edmund Halley realised in 1705 that repeated sightings of a
comet were in
fact recording the same object, returning regularly once every 75-6 years. This
proved once and for all that comets were not atmospheric phenomena, as had been
previously thought, and was the first evidence that anything other than the
planets orbited the Sun.
William Herschel was looking for binary stars in the constellation of
Taurus when he
observed what he thought was a new comet. In fact, its orbit revealed that it
was a new planet,
Uranus, the first ever discovered.
Giuseppe Piazzi discovered
Ceres in 1801, a small world between Mars and Jupiter that was initially
considered a new planet. However, subsequent discoveries of thousands of other
small worlds in the same region led to their eventual separate reclassification:
By 1846, discrepancies in the orbit of Uranus led many to suspect a large
planet must be tugging at it from farther out.
Urbain Le Verrier's calculations eventually led to the discovery of
Further discrepancies in the orbits of the planets led
Percival Lowell to conclude yet another planet, "Planet
X" must still be out there. After his death, his Lowell Observatory
conducted a search, which ultimately led to
Clyde Tombaugh's discovery of
Pluto in 1930.
Pluto was, however, found to be too small to have disrupted the orbits of the
outer planets, and its discovery was therefore coincidental. Like Ceres, it was
initially considered to be a planet, but after the discovery of many other
similarly sized objects in its vicinity it was eventually reclassified as a
In 1992, astronomers
Jewitt of the
University of Hawaii and
Jane Luu of
Massachusetts Institute of Technology discovered
This object proved to be the first of a new population, which came to be known
Kuiper Belt; an icy analogue to the asteroid belt of which such objects as
Pluto and Charon were deemed a part.
David Rabinowitz announced the discovery of
Eris in 2005, a
Scattered disc object larger than Pluto and the largest object discovered in
orbit round the Sun since Neptune.
Observations by spacecraft
Since the start of the
a great deal of exploration has been performed by
unmanned space missions that have been organized and executed by various
space agencies. The first probe to
land on another solar system body was the
2 probe, which impacted on the Moon in 1959. Since then, increasingly
distant planets have been reached, with probes landing on
Venus in 1965,
Mars in 1976, the asteroid
433 Eros in
Titan in 2005. Spacecraft have also made close approaches to other planets:
Mercury in 1973.
The first probe to explore the outer planets was
which flew by
Jupiter in 1973.
was the first to visit
Saturn, in 1979. The
Voyager probes performed a grand tour of the outer planets following
their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in
1980 – 1981.
then went on to make close approaches to
Uranus in 1986 and
Neptune in 1989. The Voyager probes are now far beyond
Neptune's orbit, and astronomers anticipate that they will encounter the
which defines the outer edge of the solar system in the next few years.
All planets in the solar system have now been visited to varying degrees by
spacecraft launched from Earth, the last being Neptune in 1989. Through these
unmanned missions, humans have been able to get close-up photographs of all of
the planets and, in the case of landers, perform tests of the soils and
atmospheres of some.
Kuiper belt object has been visited by a spacecraft. Launched on
19 January 2006, the
Horizons probe is currently enroute to becoming the first man-made
spacecraft to explore this area. This
unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove
feasible, the mission will then be extended to observe a number of other Kuiper